Color filters have a number of applications in which control of the spectral characteristics of a light source is desirable. For example, color filter arrays are employed in combination with image sensors to define color images or in combination with display devices to permit color images to be viewed. Whenever the term "image sensor", is used in the specification or claims, it will be understood to include display devices. A common approach to producing color filters has been to use organic dyes embedded in a layer. This approach has two significant disadvantages. The spectral characteristics of the filter are controlled by the absorbance curves of the dye and layer materials. Altering the spectral characteristics, therefore, requires altering the dye or layer material, which can be a difficult and time-consuming process. Furthermore, the dyes may be subject to fading with time, especially under harsh environmental operating conditions such as high light levels, etc.
An alternative, which overcomes the disadvantages of the organic-dye approach, has been to produce color filters from interference filters made up of alternate layers of two dielectric materials with different refractive indices, For purposes of this disclosure, the term "dichroic" will be understood to refer to a filter which has alternate layers of dielectric materials with different refractive indices. Various combinations of pairs of dielectric materials and deposition techniques have been used. One particular interference filter uses refractory oxides (i.e., silicon dioxide and titanium dioxide). These materials are deposited by evaporation or sputtering techniques. This is not only an expensive process, but to control the filter spectral characteristics, only the thickness of the layer can be changed. Another problem associated with image sensors, is that the materials are usually deposited at significantly greater temperatures than desirable for final manufacturing step normally associated with sensors. Excessive heating may damage the metallization used for electrical conduction in the image sensor. Therefore, the general approach has been to fabricate filters from these materials onto glass substrates and then attach the filters to devices using adhesives. Pattern transfer techniques may also be required, making the process complicated and cumbersome. Lower temperature deposition techniques for these materials are not known.
It is, of course, highly desirable to integrally make dichroic filters on an image sensor. Also, the number of processing steps in critical process controls should be minimized. Wherever possible, etch chemistries should be eliminated. It should be noted that both wet and dry etching techniques require many process controls and have to be monitored and maintained to provide an effective and safe manufacturing environment.
Another method of making multilayer optical filters using multilayer structures formed by heat treating sol-gel solutions. The filter is annealed each time a layer is applied. See Partlow et al, Thirty-seven Layer Optical Filter From Polymerized Sol-gel Solutions, Vol. 29, No. 10, Applied Optics (April 1990) and U.S. Pat. No. 4,476,156 to Brinker et al. This method is effective for making filters, but here again it is difficult to control filter spectral characteristics.